As disclosed in U.S. Pat. No. 4,746,981, J. Nadan, E. Bahr and P. Noble, assigned to the assignee of the present application, when video signals are applied to a conventional television set or monitor, the size of the image is limited to the size of the display screen of the television set or monitor, and the capacity for providing special effects is limited. Thus, FIG. 1a depicts an image in the form of the letter A on the screen 10 of one television receiver or monitor, in response to the reception of video signals corresponding to this image. It is furthermore possible to physically combine a plurality of television receivers or monitors, to form a larger image. As illustrated in FIG. 1b, four rows of four television receivers or monitors each have been stacked, to provide separate images on their respective display screens 11-26 respectively, of the letter A, in response to same video signal. It is further possible to modify the video signal corresponding to the letter A for application to the television receivers or monitors of FIG. 1b, so that the composite image on the array of their display screens forms an enlarged image, as illustrated in FIG. 1c. This effect can be created by modifying the signals to apply signals to the different receivers or monitors corresponding to different portions of the image.
Images of the type shown in FIGS. 1b and 1c are advantageous, for example, for visual merchandising, advertising, trade shows, etc.
The enlargement of an image to be displayed by a factor N requires that the number of lines from the video input signal applied to each receiver or monitor be divided by a factor of N. For example, if N equals 4, and the original video signal corresponds to 525 lines, of which about 480 are conventionally displayed, the signal is to be broken down a factor of 4, such that only 120 of the original input lines are displayed on each receiver. Further, only 1/Nth of each line is to be displayed on each receiver. This breakdown of the signals and the portion thereof displayed on each display screen is illustrated in FIG. 2. The breakdown may be effected, for example, by applying the composite video signal to a memory, with each receiver addressing the memory to recover only a determined portion of the stored data. In such an arrangement it is of course desirable to repeat each line or portion of a line N times in succession, where N is the number of vertically stacked display screens, in order to avoid an excessive number of blank lines on the individual display screens.
In the illustrated example, each display screen displays only a quarter of the lines of the video signals that formed the original image, and hence the signal modification requires the selection of the required part of the video line, and stretching it timewise to extend across the full width of the respective display screen.
Unless special care is taken in the modification of the signal for application to each TV set or monitor, a picture will result that has visual artifacts that are noticeably annoying. These artifacts result from the fact that the transmitted picture or frame is in the form of two interlaced fields, i.e., all of the odd number lines, corresponding to the first field, are applied in a first time period of for example one sixtieth of a second, and all of the even numbered lines of the image, corresponding to a second field, are sent in the next successive time period of, for example, one sixtieth of a second. This effect is illustrated in FIG. 3, for the representative example of lines 100 through 108 of an image corresponding to the video signals. Such a signal is said to be "interlaced" since the two fields are displayed spatially within each other, the two one sixtieth of a second fields being employed to build the total picture or frame in one thirtieth of a second.
When the video signals are "expanded" to display the image on a plurality of display screens, it is apparent that the numbers of the lines (i.e. numbering downward from the top line of a frame) do not directly correspond to the line numbers of the overall image presented by the plural display screens. Thus, since each of the display screens has the same number of lines as those represented in the original video signals, it is apparent that the total number of lines of all the plural display screens is N times the number of lines of the original signal, wherein N is the number of vertically stacked display screens. Assuming for example that four display screens are stacked vertically, then without further steps being taken it is apparent that each line of the original video signals will be displayed four lines apart on the composite display screen, in order to be positioned correctly (omitting for the moment discussion of the contents of the remainder of the lines). Thus, considering the interlacing effect in an expanded picture, again with four vertically stacked display screens in the composite image, it is apparent that the lines r in the original image correspond to lines R in the expanded image, in accordance with the following relationship: EQU R.sub.odd =4r.sub.odd -3 EQU R.sub.even =4r.sub.even -6
Thus, in this example, lines 100 through 108 in the image of the original video signals correspond to lines in the range of 394 to 426 in the expanded image, in the manner illustrated in FIG. 4.
An expanded image formed in accordance with the technique of FIG. 4 is unsatisfactory since video signals are displayed only on a fraction of the lines, i.e., one fourth in the illustrated example. In order to overcome this problem, it is of course possible to repeat each video signal line four successive times, in the respective field, for example in the manner illustrated in FIG. 5. In this example, original line 100 of one field has been reproduced at lines 394, 396, 398 and 400 of the expanded field, while line 101 of the other field has been displayed on lines 395, 397, 399 and 401 of the expanded field. It is apparent, however, that the expanded image illustrated in FIG. 5 will be strongly visibly impaired because the vertical spatial relationships in the original picture are destroyed by the interlacing in the expanded picture. Thus, it is apparent in FIG. 5 that the vertical relationship between the lines of the two fields is not maintained, the information of video signal line 101 correctly following the video signal line 100 in the original image, but appearing before video information corresponding to line 100 on several occasions the expanded image. This effect is vertically incorrect and noticeably annoying to the viewer.
In a solution to this problem, in accordance with the disclosure of U.S. Pat. No. 4,746,981, this vertical distortion may be overcome by blanking every fourth line in the odd fields, and blanking out the first two of every four lines of the even fields, of the enlarged image. This is illustrated in FIG. 6, wherein the lines of the odd and even fields that are displayed are shown as dots, whereas the lines that are blanked are indicated by the letter B. In this illustration, each three successive lines of the odd field replicates the same video line, and the sequences of two even video lines that follow one another replicate the same line from the even video fields. As a result, in the expanded image, only three lines are omitted in each group of eight successive lines representing an odd field video line and the next successive even field video line of the original image. It is further evident that the technique illustrated in FIG. 6 maintains the correct order of information of lines of the odd and even fields.
The correspondence between the original video line and the lines of the expanded image, in accordance with the arrangement of FIG. 6, are shown in FIG. 7, FIG. 7 clearly showing that the first two lines of the odd field are blanked and the last line of the even field is blanked, in each sequence of eight lines in the expanded image. Thus, a non-visual distorted picture may be provided by selectively blanking of the multiply replicated lines of the odd and even fields. Other combinations of blank lines and replicated lines may be employed to produce the same effect, within the scope of the invention.
FIG. 8 discloses one arrangement that may be employed to modify the video signals in accordance with the disclosure of U.S. Pat. No. 4,746,981, for display on multiple display screens, is illustrated in FIG. 8. In this arrangement, a composite video signal CV of conventional nature, for example a color signal in accordance with NTSC requirements, is applied to an NTSC decoder, synchronization signal detector and clock generator 50. This circuit processes television video signals in the conventional manner, to produce, for example, red, green and blue color signals (Ri, Gi, Bi), a clock signal Cl, and vertical and horizontal synchronization signals H and V. The color signals are applied to separate low pass filters 51, the drawing hence illustrating three such filters. The filters prepare the signals for digitization to prevent aliasing. Separate filtered color signals are then applied to separate analog to digital converters 52, the figure illustrating three such converters. The digitization rate is determined by the clock C1 from the clock generator, and is of a rate adequate to digitize signals of the frequency of the video signals, in accordance with conventional practice. The generation of the clock signals in the circuit 50 may of course be effected in accordance with conventional practice.
The digitized color signals are then applied to memories 53. A memory system is provided for each of the display screens in the expanded display, for example 16 memory systems in the above discussed example employing 4 rows of vertically stacked display screens, each having 4 display screens. Further, each memory system preferably incorporates three memories for storing the separate color signals of odd fields, and three memories for storing the separate color signals of even fields. It is of course apparent that for a black and white display only a pair of memories, for the odd and even fields, need be employed.
The read output of the memory systems for corresponding display screens are directed to separate digital to analog converters 53, and then to separate low pass filters 54, to produce the output color signals (Ro, Go, Bo) for application to the respective separate display screens. Thus, in the illustrated example, each read out color signal for each display screen is processed through a separate digital to analog converter and a separate low pass filter. The read/write control and addressing of the memories 53 is effected by an address and clock generator 55, the address and clock generator providing address signal A for the memories, and clock signals C2 for the memories and the converters 53. Addressing clock pulses are synchronized with the vertical and horizontal synchronization signals as well as with the clock signal from the circuit 50.
One method for writing data in the memories, and reading data therefrom, is illustrated in FIG. 9, wherein the top and bottom horizontal lines of the figure illustrate the composite video input signal CVin, with the sequential odd and even field data. The video information of the odd fields is written in the odd field memory, and the video information from the even fields is written into the even field memory. The data corresponding to the odd fields is read from the odd field memory at a time delayed one field time from when it was written, and, similarly, data in the even fields is read from even field memory at a time delayed by one field from the time when this data was written in the memory. By employing this double buffering technique, each memory is always being either written to or read from, but not simultaneously.
The method of detecting odd or even fields, as disclosed in U.S. Pat. No. 4,746,981, is based upon the standard NTSC format, in which the starting line of the image of one field occurs an integral number (16) of horizontal pulses following the vertical synchronization pulse, while the first horizontal pulse corresponding to a line to be displayed occurs at a time corresponding to an integral number (16) plus one half horizontal line period following the vertical synchronization pulse. It is hence only necessary to count horizontal pulses following the vertical synchronization pulse to determine whether the current video information is derived from an odd or an even field.
The video information is stored in the memory system of each display screen in accordance with the sequence of signals to be displayed on the respective display screen, so that the data may be readily sequentially read out from the rows of the odd and even field memories of that display screen. It will of course be apparent that other memory storage techniques may alternatively be employed, employing different memory read out techniques. In the preferred technique, however, after the detection of the reception of data corresponding to an odd field, the video information of the first line of the video signal is replicated in the first three rows (or the equivalent thereof) of the odd field memory, the fourth row thereof being left blank. Similarly, the video information from the third video line of the original video signal is replicated in the next three rows of the odd field memory, with the following row being left blank, etc. Upon the detection of the reception of video information from an even field, the video information of the first line is replicated in the third and fourth rows of the even field memory with the first two rows being left blank. Then, video information from the second line of the even field video signal is replicated on the seventh and eighth row of the even field memory, with the fifth and sixth rows left blank. This technique is illustrated in FIG. 10. When the data is stored in this manner, the even and odd field memories may read out on a row by row basis, to produce the output video information directly for the respective display screen. The lines of original video information that are stored in each memory correspond only to those lines that will be displayed on the given display screen, the representation of FIG. 10 thus being correct only for the top row of display screens when the display screens are stacked four high in the vertical direction. Data stored for display screens of different vertical levels must have correspondingly different sequences of lines of the original video signal.